The long-term goal of this research is to understand the neurophysiological basis for central and peripheral neurological disorders of postural control in order to provide a scientific basis for the evaluation and rehabilitation of balance disorders in the elderly. The first specific aim is to investigate the extent to which compensatory stepping is controlled in a similar way to step initiation by (1) testing the hypothesis that disorders of voluntary step initiation in 10 Parkinson's patients, 10 healthy young and 10 healthy elderly controls are similar to other disorders in stepping for balance correction (Experiment 1), and (2) determining the effect that a cognitive task has on voluntary step initiation, compensatory stepping, and feet in-place responses in 10 Parkinson patients ON and OFF levodopa and 10 healthy elderly controls (Experiment 2). The second aim investigates the effects of foot placement in stance on postural stability to perturbations in multiple directions. The hypothesis will be tested that 10 Parkinson, 10 cerebellar and 20 healthy elderly controls will not differ in their responses to such perturbations when standing with narrow, wide and diagonal foot placements (Experiment 3). Finally, the third aim is to determine how a cane is used for stability in stance and during surface perturbations. The hypothesis will be tested that a cane can be used to stabilize sway in stance, to modify and trigger automatic postural responses in patients with 10 peripheral neuropathy patients and up to 30 healthy controls, but not in 10 Parkinson's patients (Experiment 4). A better understanding of adaptive postural behaviors such as protective stepping, alterations in foot placement in stance, and use of a cane will provide the scientific rationale for improving balance retraining programs for both neurological patients and for elderly with subclinical disorders affecting balance.